Hot-melt coating: Water sorption behavior of excipient films

Hot-melt coating allows encapsulation of water-labile, drug-laden substrate s to form a barrier that resists moisture ingress. To understand the intera ction of water with excipients that can form moisture-protective coatings, sorption behavior of films of lipidic (glyceryl behenate) and polymeric (po lyvinyl alcohol) coating excipients was investigated. A simple and rapid me thod using a new, fully automated instrumental technique to investigate the sorption/desorption behavior of excipient films is reported. Further, the influence of temperature and film thickness on the sorption behavior of fil ms is examined. Both excipient films displayed sorption isotherms that were classified as type III and demonstrated hysteresis during desorption. The sorption data for both films did not follow the Langmuir model, and the BET model could only be used restrictively. The GAB model fitted the sorption data at all conditions and over the entire range of water activity studied. The ability of the Young and Nelson model to explain the hysteresis behavi or, from analytical and mechanistic perspectives, is evaluated. Temperature and film thickness were found to profoundly influence the nature of moistu re interaction and distribution of moisture in the excipient films. An Arrh enius-type relationship was observed between equilibrium moisture content o f excipient films and temperature at constant water activity.